, Volume 48, Issue 1-2, pp 1-18

Sensitivity studies on the calculation of the radiation balance of urban surfaces: II. Longwave radiation

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The model developed in Verseghy and Munro (1989) is extended to the calculation of the longwave radiation incident on building surfaces. When compared with field measurements, the average magnitude of error associated with model predictions is found to be 10 W m−2. The effects of six simplifying assumptions are investigated. The neglect of horizon obstructions is found to lead to errors of up to 60 W m−2; the assumption of wall temperatures equal to air temperatures results in errors of up to 35 W m−2. The neglect of absorption and emission by air between pairs of walls causes errors of the same magnitude as those associated with the predictions of the rigorous model itself. Of the three remaining simplifying assumptions tested (the assumption of isotrophic sky radiation, the use of published values of emissivities instead of measured values, and the blackbody surface assumption), none results om errors >5 W m−2. As in the shortware case, the errors are site-specific, but nevertheless indicate the care with which the use of simplifying assumptions must be approached.